Two suicide car bombers attacked the offices of two mobile phone operators on Saturday in Nigeria's northern city of Kano, killing themselves but no civilians, police said.

"There has been a bomb blast at Airtel office. From what we hear, it was a suicide attack. The bomber rammed his car into the gate and blew himself up. The building is on fire," a policeman told the AFP news agency on condition of anonymity.

South Africa's MTN was also targeted.

"The one who hit the Airtel office was shot by military men before the bomb exploded ... at the MTN office the car rammed into the fence but no civilians were killed," Ibrahim Idris, the chief of police in Kano, told Reuters by phone.

Islamist sect Boko Haram has previously blown up telephone masts and offices of phone companies, saying they help the security forces catch its members.

A military source said one security guard was injured and has been taken to hospital.

MTN and Airtel Nigeria's parent company Bharti Airtel, India's top cellphone operator, gave no immediate comment.

The national emergency agency confirmed the bombings and said it was not aware of any civilian casualties.

The security forces have played down the death toll in previous bombings.

At least 2,800 people have died in fighting in the largely Muslim north since Boko Haram launched an uprising against the government in 2009, watchdog Human Rights Watch says.

The sect wants to impose strict Islamic law on a country of 160 million people split roughly equally between Christians and Muslims.

The group has previously targeted churches on Christmas Day and security has been increased in all the major northern cities, although security experts say given the scale of Christian worship in Nigeria they cannot protect everyone.

Kano, Nigeria's second-largest city after the southern commercial hub Lagos, was the site of Boko Haram's deadliest attack which killed at least 186 people in January in coordinated bombings and shootings.

Armed police have been guarding major churches in Kano this week and additional police checkpoints have been set up around the majority-Muslim city, a Reuters witness said.

Police in Kano said this week that their anti-terrorism squads have been searching houses and buildings they suspect to be hideouts of criminals and "terrorists".

Security experts say they believe Boko Haram is seeking to spark a religious conflict by targeting Christians in a countrywhere ethnic violence has flared up periodically in recent years, in some cases killing hundreds in the space of hours.

A French national was kidnapped in far northern Nigeria, close to the border with Niger, this week by people France's intelligence agency said were "an organised group linked to a terrorist activity".